I had pair of Spendor D7 which uses a quasi-transmission line bottom firing port. The bass and sub-bass was so deep and effortless, a speaker with very special bass. It improves the efficiency of bass response and makes them easier to position.
My issue with the Spendors is that they were voiced nearly flat without the usual midbass bump. Otherwise they would have been perfect.
T-line designs have the potential to be outstanding. Application is unfortunately not consistent.